Once a lender has issued a 1099-C, it is against the regulations for them to continue collection efforts. You could try reporting them to your state banking regulator if that's what is happening.
The 1099-C is supposed to give the date of the "identifiable event" in box 1. This is the date that the lender recognizes as the date the debt officially became uncollectible. It could be the date of the foreclosure or just the date they gave up. If the date is in 2023, you can report it on your current tax return. There is a procedure where, if the debt was due to foreclosure on the home you lived in as your main home at the time, the forgiven debt does not have to be taxable. But you need to report it and work through the interview.
If the date in box 1 is for a date before 2023, then you can't report it this year, you would need to report it on an amended return for the correct year. You may want to see a tax professional. There are some (fairly tricky) rules on 1099-Cs, and issuing a 1099-C 15 years after the foreclosure may be improper. But you would need someone to help you with that.
